1. Field of the Invention
The invention is in the field of joining cast ductile iron pipe. More specifically, it is in the field of joining ductile iron pipe in such a manner that they will not separate when subjected to axial loads.
2. Description of the Prior Art
It is common practice to make pipe joints wherein one pipe having a spigot or plain end is pushed into the bell end of another pipe. To prevent leakage a soft rubber ring with a hard rubber insert on one side is placed into the bell of the outside pipe, prior to the insertion of the plain end.
To prevent the pipes from separating under pressure such joints have been modified by using tightening bolts, and tightening plates to connect the pipes. However, these plates and bolts have proven to be costly and complicated to use in the field.
A newer method of connecting such pipes is taught by U.S. Pat. No. 3,684,320. In this patent the outer circumference of the plain end of one pipe has a series supporting elements and the bell end of the other pipe has at least one projection, protruding downwardly towards the inside of the bell. The diameter of the projection is at least equal to the diametric distance between the outer surfaces of the supporting elements so that the plain pipe with its supporting elements can be pushed into the bell end of the other pipe and inwardly beyond the projection. Locking members are inserted between the projection and the supporting elements to lock the pipe together. The locking members are introduced into a recess or recesses in the projection and rotated to engage a portion of the projection and the supporting elements.
In the disclosure of the prior art Patent U.S. Pat. No. 3,684,320 and in actual practice the mating surface of the projection and the locking members have the shape of a spherical shell and both spherical shells have the same diameter. In effect the two surfaces coact in a wedging action so that as the inner and outer pipes are moved apart by the pressure of the internal fluid, the two mating surfaces slide against each other. Since the projection has a slope slanting downwardly in the direction of the open face of the bell and the locking segments have a slope in the same direction, as the pipes tend to move apart, a compressing action takes place between the projection and the locking segment because the locking segment is held tightly against axial movement by the supporting elements.
The compressing action tends to deform the plain end of the inner pipe and will cause the pipe to break, if sufficient radially inward force is applied.
While structures such as those shown in U.S. Pat. No. 3,684,320 provide a means to overcome axial loads on joints of ductile iron pipe up to about thirty-six inches in diameter, since the axial load varies with the square of the diameter, extremely high axial loads must be resisted, when ductile iron pipes of greater diameter are installed. For example, the end load at 500 p.s.i. on a 24 inch inside diameter cylinder with capped ends is about 226,000 pounds, while at the same pressure a 48 inch inside diameter cylinder would have an end load of about 905,000 pounds. The structure of U.S. Pat. No. 3,684,320 allows certain loads to become excessive when his method is used on very large cast ductile pipe.